This document uses the following terms:
Explicit FTPS: A common term for an FTPS implementation based on [RFC4217]. It refers to the fact that an explicit protocol handshake is required to promote a FTP connection (most commonly connected to port 21) from a non-secure to secure one.
File Transfer Protocol (FTP): A member of the TCP/IP suite of protocols that is used to copy files between two computers on the Internet if both computers support their respective FTP roles. One computer is an FTP client and the other is an FTP server.
FTPS: The File transfer Protocol over SSL/TLS Extension [RFC4217] of [RFC959], which enables the secure transfer of information between client and server.
Implicit FTPS: A common term for an early implementation of FTPS (based on the now-expired Internet draft [EXPIRED-FTP-DRAFT]) that required a dedicated port that would be used exclusively for the SSL/TLS protected data transfer. TLS/SSL would be negotiated immediately after a TCP connection was established. It is analogous to HTTPS protocol handling [RFC2818].
Implicit SSL: See Implicit FTPS.
MAY, SHOULD, MUST, SHOULD NOT, MUST NOT: These terms (in all caps) are used as defined in [RFC2119]. All statements of optional behavior use either MAY, SHOULD, or SHOULD NOT.